Method of manufacturing casting for mold for molding tire

ABSTRACT

Provided is a method of manufacturing a casting for a mold for molding a tire, wherein twist and warp deformation of each block of the casting is less likely to occur when the casting shrinks, the difference between the degrees of shrinkage of cope and drag is small, the casting, even if large-sized, can be produced in a relatively short molten metal solidifying time, and healthy castings can be easily obtained. A method of manufacturing a casting for a mold for molding a tire of a sectional mold type in which the mold is opened and closed by dividing the mold into a plurality of pieces in the circumferential direction comprises a process of manufacturing each of divided block castings ( 1 ) by casting them individually. In this process, molten metal is poured into a casting mold where chills are disposed on four sides, the sides being an upper surface portion ( 2   a ), a lower surface portion ( 2   b ) and both sides of circumferentially divided surfaces ( 3   a,    3   b ), and surrounding from four directions a design surface ( 6 ) which is a contact surface with the mold from four directions, such that at least the chills surround the design surface ( 6 ) continuously.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a casting for a mold for molding a tire of a sectional mold type, and particularly to a method of manufacturing a casting for a mold for molding a tire, wherein twist and warp deformation of each block of the casting is less likely to occur when the casting shrinks, the difference between the degrees of shrinkage of cope and drag is small, the casting, even if large-sized, can be produced in a relatively short molten metal solidifying time, and healthy castings can be easily obtained.

BACKGROUND ART

Molds for molding a tire are usually produced by a casting process because the design of the molds is intricate, and the molds have the property that the molds enclose thin plates such as sipes and blades made of different kinds of metal materials. Plaster casting is widely adopted as a method of manufacturing a casting for a mold for a tire. Other reasons why plaster casting is adopted are

(1) that castings having a melting point of up to about the melting point of an aluminum alloy can be manufactured with a high dimensional accuracy;

(2) that cutting process and assembly process can be easily performed at a stage of a plaster casting mold;

(3) that the plaster casting can flexibly deal with inserts for sipes or blades; and

(4) that an intricately designed shape can be accurately replicated by reverse casting from a rubber molding; or the like.

A mold dividing structure of a mold for a tire includes two types, that is, a two-piece mold in which the mold is divided into two pieces in the tire width direction and a sectional mold in which the mold is divided into 7 to 11 pieces in the tire circumferential direction. Among these structure, a sectional mold which has a low resistance at the time of molding and demolding of a tire and has a high dimensional accuracy is widely used. As the method of casting a sectional mold, for example, Patent Document 1 discloses a method of pouring molten metal by a low-pressure casting method, Patent Document 2 discloses a method of pouring molten metal by gravity casting using a tub for the exclusive use (chute), and Patent Document 3 discloses a method of pouring molten metal by gravity casting using a reusable runner-board.

Examples of characteristics of these method include (1) that, after the casting, the mold is processed into a sectional mold; (2) that the molten metal solidifying time differs between the upper portion and the lower portion per one sectional mold unit because tap holes are concentrated at the lower surface side, and (3) that a casting flask is applied as a chill.

FIG. 19 illustrates an example of a process of manufacturing a mold for a tire by a sectional mold. (A) is a plan view of a ring casting 100 of a mold for a tire, and (B) is the cross sectional view thereof. In this example of the process, first, a mold for a tire is casted, and then divided into individual block castings 101 by dividing into sectors, and thereafter, the periphery of the casting is processed to obtain a sectional mold. In this method, both simultaneous casting of a plurality of block castings in a ring shape and casting of a block casting one by one are applicable.

PRIOR ART DOCUMENT Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Application     Publication No. 57-58968 (claim or the like) -   Patent Document 2: Japanese Patent No. 2796010 (claim or the like) -   Patent Document 3: Japanese Unexamined Patent Application     Publication No. 2007-144480 (claim or the like)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Casting a casting for a mold for a tire by a sectional mold has advantages that twist and warp deformation of sectional mold block casting is less likely to occur when the casting shrinks, and that a casting productivity is high because the casting for a mold for a tire can be casted with one ring. However, in a conventional sectional mold, since a difference of molten metal solidifying time between the upper surface and the lower surface occurs and casting shrinkage increases at the portion where solidification is slow because a tap hole is on the side of the lower surface or the upper surface, there is a problem that a size difference between cope and drag is likely to occur.

Accordingly, an object of the present invention is to provide a method of manufacturing a casting for a mold for molding a tire, wherein twist and warp deformation of each block of the casting is less likely to occur when the casting shrinks, the difference between the degrees of shrinkage of cope and drag is small, the casting, even if large-sized, can be produced in a relatively short molten metal solidifying time, and healthy castings can be easily obtained.

Means for Solving the Problems

To solve the above-described problems, the present inventor intensively studied to discover that the above-mentioned object may be attained by the following constitution, thereby completing the present invention.

That is, the method of manufacturing a casting for a mold for molding a tire of the present invention is a method of manufacturing a casting for a mold for molding a tire of a sectional mold type in which the mold is opened and closed by dividing the mold into a plurality of pieces in the circumferential direction, said method comprising

a process of manufacturing each of divided block castings by casting them individually, in which process, molten metal is poured into a casting mold where chills are disposed on four sides, the sides being an upper surface portion, a lower surface portion and both sides of circumferentially divided surfaces, and surrounding from four directions a design surface which is a contact surface with the mold, such that at least the chills surround said design surface continuously.

In the present invention, it is preferred that opposing said chills be respectively disposed symmetrically on said four sides, the sides being an upper surface portion, a lower surface portion and both sides of circumferentially divided surfaces. It is also preferred that a pair of tap holes to said casting mold be disposed symmetrically on said upper surface portion and lower surface portion, and a pair of tap holes to said casting mold be disposed symmetrically on said both sides of circumferentially divided surfaces. Further, it is preferred that a tap hole and/or a runner which provides the tap hole with molten metal be formed inside said chill.

Effects of the Invention

By the present invention, a method of manufacturing a casting for a mold for molding a tire may be provided, wherein twist and warp deformation of each block of the casting is less likely to occur when the casting shrinks, the difference between the degrees of shrinkage of cope and drag is small, the casting, even if large-sized, can be produced in a relatively short molten metal solidifying time, and healthy castings can be easily obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a block casting of one embodiment of the present invention.

FIG. 2 is a perspective view illustrating a block casting of another embodiment of the present invention.

FIG. 3 is a perspective view illustrating a method of manufacturing a casting for a mold for molding a tire of another embodiment of the present invention.

FIG. 4 is a perspective view illustrating a method of manufacturing a casting for a mold for molding a tire of another embodiment of the present invention.

FIG. 5 is a perspective view illustrating a method of manufacturing a casting for a mold for molding a tire of another embodiment of the present invention.

FIG. 6 is a perspective view illustrating a method of manufacturing a casting for a mold for molding a tire of another embodiment of the present invention.

FIG. 7 is a perspective view illustrating a method of manufacturing a casting for a mold for molding a tire of another embodiment of the present invention.

FIG. 8 is a perspective view illustrating a method of manufacturing a casting for a mold for molding a tire of another embodiment of the present invention.

FIG. 9 is a perspective view illustrating a method of manufacturing a casting for a mold for molding a tire of another embodiment of the present invention.

FIG. 10 is a perspective view illustrating a method of manufacturing of Example 1.

FIG. 11 is a perspective view illustrating a method of manufacturing of Example 2.

FIG. 12 is a perspective view illustrating a method of manufacturing of Example 3.

FIG. 13 is a perspective view illustrating a method of manufacturing of Comparative Example 1.

FIG. 14 is a perspective view illustrating a method of manufacturing of Comparative Example 2.

FIG. 15 a drawing for explaining the measurement of the chord size.

FIG. 16 a drawing for explaining the measurement of twist.

FIG. 17 a drawing for explaining the measurement of circumferential warp.

FIG. 18 a drawing for explaining the measurement of width direction warp.

FIG. 19 is a process drawing illustrating a process of manufacturing a mold for a tire by a sectional mold by a conventional method.

MODES FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described by way of the drawings.

Embodiments 1 to 3

FIG. 1(A) is a perspective view illustrating a block casting 1 of the first embodiment of the present invention. In order to increase the cooling speed inside the block casting 1, chills are disposed on about the half of the design surface 6 side of the upper surface portion 2 a, about the half of the back surface 4 side of the lower surface portion 2 b, about the half of the back surface 4 side of the one side of circumferentially divided surface 3 a of both sides of circumferentially divided surfaces, and about the half of the back surface 4 side of the other side of circumferentially divided surface 3 b of both sides of circumferentially divided surfaces of the block casting 1. In the figure, a portion with which a chill is in contact is marked with diagonal lines. In the present invention, as shown in the figure, it is important that a casting mold is formed such that a chill surrounds a design surface 6 continuously. By this, the portion solidifies quickly at the time of solidification of molten metal after casting, and the solidified portion functions as a hamper when the whole block casting 1 solidifies and is cooled to shrink. As the result, twist and warp deformation of the manufactured block casting 1 is less likely to occur, and the difference of degrees of shrinkage between the upper surface portion 2 a and the lower surface portion 2 b becomes small.

Since, if a mold for a tire is ring casted by a conventional sectional mold as shown in FIG. 19, a dead head hole portion needs to be formed either on the upper surface portion or on the lower surface portion, and a chill cannot be in contact with a circumferentially divided surface because of its absence, a mold cannot be formed such that a chill surrounds a design surface continuously. Therefore, in the conventional sectional mold, although the ring structure of a casting for itself minimizes the occurrence of twist and warp deformation when the casting shrinks, solidification at the side of the dead head delays and an increase in the difference between the degrees of shrinkage of the upper surface portion and the lower surface portion is inevitable.

In the present invention, as for the back surface 4 side which is the opposing surface of the design surface 6, a disposition of chills for the block casting 1 is not particularly restricted. A chill may be in contact with the whole surface except for a hole portion of the dead head 5, or a chill may not be disposed and only a molding box may be used.

FIG. 1(B) is a perspective view illustrating a block casting 1 of the second embodiment of the present invention. Chills are disposed on the whole surface of the upper surface portion 2 a, the whole surface of the upper surface portion 2 b, about the half of the design surface 6 side of the one side of circumferentially divided surface 3 a of both sides of circumferentially divided surfaces, and about the half of the back surface 4 side of the other side of circumferentially divided surface 3 b of both sides of circumferentially divided surfaces of the block casting 1. FIG. 1(C) is a perspective view illustrating a block casting 1 of the third embodiment of the present invention. Chills are disposed on about the half of the design surface 6 side of the upper surface portion 2 a, about the half of the back surface 4 side of the lower surface portion 2 b, the whole surface of both sides of circumferentially divided surfaces 3 a, 3 b of the block casting 1. Also in these embodiments, in the same manner as in the first embodiment, the chills can surround the design surface 6 continuously, and as the result, the portion solidifies quickly at the time of solidification of molten metal after casting, and the solidified portion functions as a hamper when the whole block casting 1 solidifies and is cooled to shrink.

Embodiments 4 to 8

In the following embodiments 4 to 8, a suitable embodiment will be described in which opposing chills are respectively disposed symmetrically on four sides, the sides being an upper surface portion, a lower surface portion and both sides of circumferentially divided surfaces of the block casting 1.

FIG. 2(A) is a perspective view illustrating a block casting 1 of the fourth embodiment of the present invention. Opposing chills are respectively disposed symmetrically by disposing the chills on the whole surface of an upper surface portion 2 a, the whole surface of a lower surface portion 2 b, and the whole surfaces of both sides of circumferentially divided surfaces 3 a, 3 b of the block casting 1.

FIG. 2(B) is a perspective view illustrating a block casting 1 of the fifth embodiment of the present invention. Opposing chills are respectively disposed symmetrically by disposing the chills on about the half of the design surface 6 side of the upper surface portion 2 a, about the half of the design surface 6 side of the lower surface portion 2 b, and about the halves of the design surface 6 side of both sides of circumferentially divided surfaces 3 a, 3 b of the block casting 1.

FIG. 2(C) is a perspective view illustrating a block casting 1 of the sixth embodiment of the present invention. Opposing chills are respectively disposed symmetrically by disposing the chills on about the half of the back surface 4 side of the upper surface portion 2 a, about the half of the back surface 4 side of the lower surface portion 2 b, and about the halves of the back surface 4 side of both sides of circumferentially divided surfaces 3 a, 3 b of the block casting 1.

FIG. 2(D) is a perspective view illustrating a block casting 1 of the seventh embodiment of the present invention. Opposing chills are respectively disposed symmetrically by disposing the chills on about the half of the design surface 6 side of the upper surface portion 2 a, about the half of the design surface 6 side of the lower surface portion 2 b, and about the halves of the back surface 4 side of both sides of circumferentially divided surfaces 3 a, 3 b of the block casting 1.

FIG. 2(E) is a perspective view illustrating a block casting 1 of the eighth embodiment of the present invention. Opposing chills are respectively disposed symmetrically by disposing the chills on about the half of the back surface 4 side of the upper surface portion 2 a, about the half of the back surface 4 side of the lower surface portion 2 b, and about the halves of the design surface 6 side of both sides of circumferentially divided surfaces 3 a, 3 b of the block casting 1. It is noted that, in each of the figures, a portion with which a chill is in contact is marked with diagonal lines.

In each of the embodiments 4 to 8, chills surround the design surface 6, and thus, the invention of the embodiments 4 to 8 can obtain the same effect as in the invention of the first embodiment. In addition, in the embodiments 4 to 8, by disposing respectively opposing chills symmetrically on four sides, the sides being an upper surface portion 2 a, a lower surface portion 2 b and both sides of circumferentially divided surfaces 3 a, 3 b of the block casting 1, the solidification of molten metal starts at about the same time from the upper surface portion 2 a, the lower surface portion 2 b and both sides of circumferentially divided surfaces 3 a, 3 b, and a size property in which the block casting 1 is symmetrical in up and down, left and right directions (arrow directions) is easily obtained. The disposition of chills on the back surface side 4 of the casting is not restricted.

As mentioned above, in order to attain an up and down, left and right (arrow directions) symmetric solidification configuration of the block casting 1, dead head 5 necessarily needs to be disposed on the back surface 4 side, and the solidification of molten metal at the back surface 4 side of the block casting 1 needs to be delayed. Accordingly, the disposition of chills on the back surface 4 side is not necessary since it is not necessary to obtain the effect of a hamper by initially solidified layer of molten metal at the back surface 4 when the whole block casting 1 solidifies and is cooled to shrink.

Embodiments 9 to 11

In the following embodiments 9 to 11, a suitable embodiment will be described in which a pair of tap holes for a casting mold are disposed symmetrically on the upper surface portion and the lower surface portion, or in which a pair of tap holes for a casting mold are disposed symmetrically on both of the circumferentially divided surfaces.

FIG. 3 is a schematic view illustrating a disposition of tap holes for a casting mold for a block casting 1 of the ninth embodiment of the present invention. In the illustrated suitable embodiment, a pair of tap holes 7 disposed symmetrically on the upper surface portion 2 a and the lower surface portion 2 b are formed on the periphery portions of the upper surface portion 2 a side and the lower surface portion 2 b side of the design surface 6. Molten metal which is poured from a pouring port 10 is supplied to these tap holes 7 by gravity casting through a nozzle 11 and one runner 8.

FIG. 4 is a schematic view illustrating a disposition of tap holes for a casting mold for a block casting 1 of the tenth embodiment of the present invention. In the illustrated suitable embodiment, in the same way as in the ninth embodiment, a pair of tap holes 7 disposed symmetrically on the upper surface portion 2 a and the lower surface portion 2 b are formed on the periphery portions of the upper surface portion 2 a side and the lower surface portion 2 b side of the design surface 6. Molten metal which is poured from a pouring port 10 is supplied to these tap holes 7 by gravity casting through a nozzle 11 and a two-way branched runner 8.

FIG. 5 is a schematic view illustrating a disposition of tap holes for a casting mold for a block casting 1 of the eleventh embodiment of the present invention. In the illustrated suitable embodiment, two pairs of tap holes 7 disposed respectively symmetrically on the upper surface portion 2 a and the lower surface portion 2 b are formed on the periphery portions of the upper surface portion 2 a side and the lower surface portion 2 b side of the design surface 6. Molten metal which is poured from a pouring port 10 is supplied to these tap holes 7 by gravity casting through a nozzle 11 and a two-way branched runner 8.

In each of the embodiments 9 to 11, it is supposed that chills surround the design surface 6 continuously, and thus the invention of the embodiments 9 to 11 can obtain the same effect as in the invention of the first embodiment. In addition, in each of the embodiments 9 to 11, a heat input (overheat) condition by molten metal from the tap hole 7 from the start of pouring to the completion of pouring can also be made uniform in up and down, left and right directions, whereby such an advantage that in the block casting 1, it becomes easy to obtain a symmetrical size property in up and down, left and right directions is obtained.

Embodiments 12 to 15

In the following embodiment 12 to 15, a suitable embodiment will be described in which both or either of a tap hole(s) and a runner(s) which supply(ies) molten metal to a tap hole(s) are/is formed inside a chill 12.

FIG. 6 is a schematic view illustrating a disposition of a tap hole for a casting mold for a block casting 1 of the twelfth embodiment of the present invention. In the illustrated suitable embodiment, a tap hole 7 is disposed on a periphery portion of the upper surface portion 2 a side of the design surface 6. Molten metal which is poured from a pouring port 10 is supplied to this tap hole 7 by gravity casting through a nozzle 11 and one runner 8. Here, in this twelfth embodiment, the tap hole 7 is formed inside a chill 12, that is, through the chill 12.

FIG. 7 is a schematic view illustrating a disposition of tap holes for a casting mold for a block casting 1 of the thirteenth embodiment of the present invention. In the illustrated suitable embodiment, a pair of tap holes 7 disposed symmetrically on the upper surface portion 2 a and the lower surface portion 2 b are formed on the periphery portions of the upper surface portion 2 a side and the lower surface portion 2 b side of the design surface 6. Molten metal which is poured from a pair of pouring ports 10 is supplied to these tap holes 7 by gravity casting through individual nozzles 11 and runners 8. Here, in this thirteenth embodiment, these tap holes 7 are formed inside a chill 12.

In each of the embodiments 12 to 13, it is supposed that a chill 12 surround the design surface 6 continuously, and thus the invention of the embodiments 12, 13 can obtain the same effect as in the invention of the first embodiment. In addition, in these embodiments, by disposing the tap hole 7 inside the chill 12, molten metal at the tap hole 7 solidifies and is cooled immediately after the completion of pouring, and thus even in the case of the twelfth embodiment that the tap hole 7 is not disposed symmetrically in the up and down direction, an advantage that the block casting 1 solidifies and is cooled uniformly in up and down, left and right directions in the same manner as in the thirteenth embodiment can be obtained.

FIG. 8 is a schematic view illustrating a disposition of a tap hole for a casting mold for a block casting 1 of the fourteenth embodiment of the present invention. The illustrated suitable embodiment is same as the twelfth embodiment except that a runner 8 is formed inside a chill 12.

FIG. 9 is a schematic view illustrating a disposition of tap holes for a casting mold for a block casting 1 of the fifteenth embodiment of the present invention. In the illustrated suitable embodiment, a pair of tap holes 7 disposed symmetrically on the upper surface portion 2 a and the lower surface portion 2 b are formed on the periphery portions of the upper surface portion 2 a side and the lower surface portion 2 b side of the design surface 6. Molten metal which is poured from one pouring port 10 is supplied to these tap holes 7 by gravity casting through a nozzle 11 and a two-way branched runner 8. Here, in this fifteenth embodiment, both tap holes 7 and the runner 8 are formed inside a chill 12.

In the embodiments 14 and 15, the effect of the invention of the above-mentioned embodiments 12 and 13 can be further enhanced, and the embodiments 14 and 15 have an advantage that mounting and demounting of an external gate for every casting can be saved.

A method of manufacturing a casting for a mold for molding a tire of a sectional mold type of the present invention is characterized by a process of manufacturing each of divided block castings by casting them individually. Other processes such as a pattern manufacturing process, a reversing process of a rubber molding, a reversing process of a plaster casting mold, a drying process of a mold, a mold shakeout process and a casting mold spotting process may be performed as required according to known methods.

EXAMPLES

The present invention will now be described in detail by way of Examples thereof.

Example 1

FIG. 10 is a perspective view illustrating a method of casting a block casting 1 of Example 1. As shown in the figure, a chill 12 was disposed on a casting flask at four sides, upper surface portion 2 a, lower surface portion 2 b and circumferentially divided surfaces 3 a and 3 b of the block casting 1 so as to surround a design surface 6 continuously. For the whole casting flask surrounding the block casting 1, a runner 8, a tap hole 7 and the chill 12, a sand mold made of water glass hardened silica sand was used. The block casting 1 was manufactured such that the rate of contact area of the upper surface portion 2 a with the chill was 80%, the rate of contact area of the lower surface portion 2 b with the chill was 50%, the rate of contact area of the circumferentially divided surfaces 3 a and 3 b with the chill was 60%, the temperatures of the casting mold, casting flask and chill at the time of casting were 25° C. and the casting starting temperature was 680° C. As an alloy of the casting for a mold for a tire, AC4C (aluminum alloy) was employed.

Example 2

FIG. 11 is a perspective view illustrating a method of casting a block casting 1 of Example 2. As shown in the figure, chills 12 were disposed symmetrically to each other on a casting flask at four sides, upper surface portion 2 a, lower surface portion 2 b and circumferentially divided surfaces 3 a and 3 b of the block casting so as to surround a design surface 6 continuously. For the whole casting flask surrounding the block casting 1, a runner 8, a tap hole 7 and the chill 12, a sand mold made of water glass hardened silica sand was used. A block casting was manufactured such that the rate of contact area of the upper surface portion 2 a with the chill was 50%, the rate of contact area of the lower surface portion 2 b with the chill was 50%, the rate of contact area of the circumferentially divided surfaces 3 a and 3 b with the chill was 60%, the temperatures of the casting mold, casting flask and chill at the time of casting were 25° C. and the casting starting temperature was 680° C. As an alloy of the casting for a mold for a tire, AC4C (aluminum alloy) was employed.

Example 3

FIG. 12 is a perspective view illustrating a method of casting a block casting 1 of Example 3. As shown in the figure, a casting flask (drag) 13 surrounding upper surface portion 2 a, lower surface portion 2 b and circumferentially divided surfaces 3 a and 3 b of the block casting, a runner 8, a tap hole 7 and a chill 12 were all made of cast iron. The back surface of casting flask (cope) of the block casting was made of water glass hardened silica sand. The runner 8 and the tap hole 7 were manufactured by engraving in the lower structure. The block casting was manufactured such that the rates of contact areas of the upper surface portion 2 a, the lower surface portion 2 b and the circumferentially divided surfaces 3 a and 3 b with the chill were 100%, the temperatures of the casting mold and casting flask at the time of casting were 200° C. and the casting starting temperature was 680° C. As an alloy of the casting for a mold for a tire, AC4C (aluminum alloy) was employed.

Comparative Example 1

FIG. 13 is a perspective view illustrating a conventional method of casting a casting for a mold for a tire of Comparative Example 1. As shown in the figure, ring-shaped runner 18 was disposed on the lower side of the ring, and a tap hole 17 which was equally divided into 6 pieces was disposed thereon. As for the rates of contact areas with the chills, the rate at the peripheral cylindrical surface of the ring casting was 100% in whole surface, the rate at the inside of the doughnut shape on the lower plane was about 40% and the rate at the upper surface portion 22 a where dead head 15 was generated was 0% (no contact with the chill). A block casting was manufactured such that the temperature of the casting mold and the casting flask was 200° C. and the casting starting temperature was 680° C. As an alloy of the casting for a mold for a tire, AC4C (aluminum alloy) was employed.

Comparative Example 2

FIG. 14 is a perspective view illustrating a method of casting a block casting 1 of Comparative Example 2. As shown in the figure, two chills 12 were disposed on a casting flask at each of four sides, upper surface portion 2 a, lower surface portion 2 b and circumferentially divided surfaces 3 a and 3 b of the block casting. The contact areas with the chill were all 30%. The block casting 1 was manufactured such that the temperatures of the casting mold and casting flask at the time of casting were 25° C. and the casting starting temperature was 680° C. As an alloy of the casting for a mold for a tire, AC4C (aluminum alloy) was employed. (Method of forming a design surface which is the contact surface with a mold)

A rubber molding was manufactured by disposing in a molding box a wooden mold on which a tread pattern was formed and by pouring a silicone rubber material into the molding box. The material of the molding box was a synthetic wood (basic degree of shrinkage setting: 11.5/1000) and the rubber molding was silicone rubber molding having a plaster lining (thickness of rubber layer: 15 mm).

By pouring, into the rubber molding, a plaster (G-1 foam plaster manufactured by Noritake Gypsum Co., Ltd.: mixing water ratio: 70%, increase by foaming: 50%), a design surface portion being in contact with a casting mold (design surface φ: 600±20 mm, tire width size: 195±30 mm, casting thickness: 70 to 100 mm, whole casting height: 300±30 mm, divided sectors: 9 pieces/1 ring) was produced. the design surface portion was used for casting to obtain block castings of Examples 1 to 3 and Comparative Example 2, and a casting for a mold for a tire of Comparative Example 1. Basic sizes of the casting for a mold for a tire, and the methods of producing the castings are shown in combination in Table 1 below.

TABLE 1 size of casting for a mold for a tire Manufacturing method design surface φ about Model material synthetic wood portion diameter 600 ± 20 mm (basic degree of shrinkage setting: 11.5/1000) tire width size 195 ± 30 mm rubber molding silicone rubber molding having plaster lining (thickness of rubber layer: 15 mm) casting thickness 70~100 mm plaster casting G-1 foam plaster manufactured mold by Noritake Gypsum Co., Ltd. (mixing water ratio: 70%, increase by foaming: 50%) overall casting 300 ± 30 mm alloy employed AC4C (aluminum alloy) height Si: 7%, Mg: 0.4%, Fe: 0.3%, Al: remainder divided sectors 9 pieces/1 ring casting method ring casting (about 40 deg) by gravity casting method, and block casting

As for the dimensional accuracy of the as-cast castings for a mold for a tire manufactured in Examples 1 to 3 and Comparative Examples 1 and 2, four items: chord size, twist, circumferential warp and width direction warp were evaluated according to the evaluation method below.

<Chord Dimension>

FIG. 15 is a drawing for explaining the measurement of the chord size. The upper chord size, center chord size and lower chord size of each of the obtained casting for a mold for a tire were measured and calculate the average of the differences between the measured sizes and the size in the drawing thereof. The difference between the upper chord size and lower chord size was also calculated. The difference between the chord size and the size in the drawing thereof was calculated by the following:

chord size difference=actual casting size−size in drawing.

When the value of the chord size difference is a positive value, the casting chord size is larger than the size in the drawing. The difference between the upper chord size and lower chord size was calculated by the following:

difference between upper chord size and lower chord size=lower chord size−upper chord size.

When the value of the difference between upper chord size and lower chord size is a positive value, the chord size of the cope is smaller.

<Twist>

FIG. 16 is a drawing for explaining the measurement of twist. The difference values between the size and the theoretical size at four points: both ends at the upper and lower portions of the block casting (A to D) were calculated by measuring the roundness of the as-cast casting. The total of the absolute values of the difference values: (|+A|+|−B|+|−C|+|+D|) was defined as a twist amount, and the amount of twist was evaluated.

<Circumferential Warp>

FIG. 17 is a drawing for explaining the measurement of circumferential warp. The amount of irregularity (X, Y) from the theoretical size of one sector block in the circumferential direction was calculated by measuring the roundness to evaluate the amount of circumferential warp (=−X or +Y).

<Width Direction Warp>

FIG. 18 is a drawing for explaining the measurement of width direction warp. The amount of irregularity (X, Y) from the theoretical size of one sector block in the width direction was calculated by measuring the R shape in the width direction to evaluate the amount of circumferential warp (=−X or +Y). The results obtained are shown in combination in Table 2.

TABLE 2 chord size difference from difference between circumferential width direction size in drawing upper and lower chords twist warp warp Comparative Example 1 0.05 ± 0.20 0.25 ± 0.10 0.15 ± 0.15  0.05 ± 0.15 −0.10 ± 0.10 (Conventional ring casting) (mm) Comparative Example 2 −0.05 ± 0.30  0.05 ± 0.15 0.30 ± 0.20 −0.05 ± 0.30 −0.05 ± 0.25 (Conventional block casting) (mm) Example 1 0.02 ± 0.20 0.10 ± 0.10 0.23 ± 0.17 −0.02 ± 0.20 −0.05 ± 0.15 (mm) Example 2 0.00 ± 0.20 0.05 ± 0.10 0.20 ± 0.15 −0.00 ± 0.20 −0.05 ± 0.15 (mm) Example 3 0.06 ± 0.15 0.00 ± 0.10 0.15 ± 0.15  0.00 ± 0.15 −0.03 ± 0.15 (mm)

Table 2 shows that in Comparative Example 1 (conventional ring casting), the properties of twist, circumferential warp and width direction warp were preferable (both the average and variation were small), while the difference between the sizes of upper and lower chords was the largest. In Comparative Example 2 (conventional block casting), the difference between the sizes of upper and lower chords was small, while the properties of twist, circumferential warp and width direction warp were not preferable.

On the other hand, chord size difference in Example 1 was smaller than that in Comparative Example 1, and twist and warp deformation in Example 1 could be suppressed more than in Comparative Example 2. That is, it is known that a block casting method may be provided in which twist and warp deformation is less likely to occur when the casting shrinks and the difference between the degrees of shrinkage of cope and drag is small. The difference between the upper chord size and lower chord size in Example 2 could be suppressed further more than in Example 1. Further, in Example 3, twist and warp were similar to that of ring casting, while the difference between the upper chord size and lower chord size could be greatly improved as compared to ring casting.

From the above, by the present invention, also by using a block casting method, a dimensional accuracy property which is similar to that of a ring casting method or is better than that of a ring casting method may be obtained, and the present invention may make the most of high flexibility of dealing with a large-sized article, which is an advantage of a block casting method.

DESCRIPTION OF SYMBOLS

-   1, 101 block casting -   2 a, 22 a upper surface portion -   2 b lower surface portion -   3 a, 3 b circumferentially divided surface -   4 back surface -   5, 15 dead head -   6 design surface -   7, 17 tap hole -   8, 18 runner -   10 pouring port -   11 nozzle -   12 chill -   13 drag -   100 ring casting 

1. A method of manufacturing a casting for a mold for molding a tire of a sectional mold type in which the mold is opened and closed by dividing the mold into a plurality of pieces in the circumferential direction, said method comprising a process of manufacturing each of divided block castings by casting them individually, in which process, molten metal is poured into a casting mold where chills are disposed on four sides, the sides being an upper surface portion, a lower surface portion and both sides of circumferentially divided surfaces, and surrounding from four directions a design surface which is a contact surface with the mold from four directions, such that at least the chills surround said design surface continuously.
 2. The method of manufacturing a casting for a mold for molding a tire according to claim 1, wherein opposing said chills are respectively disposed symmetrically on said four sides, the sides being an upper surface portion, a lower surface portion and both sides of circumferentially divided surfaces.
 3. The method of manufacturing a casting for a mold for molding a tire according to claim 1, wherein a pair of tap holes for said casting mold are disposed symmetrically on said upper surface portion and lower surface portion.
 4. The method of manufacturing a casting for a mold for molding a tire according to claim 1, wherein a pair of tap holes for said casting mold are disposed symmetrically on said both sides of circumferentially divided surfaces.
 5. The method of manufacturing a casting for a mold for molding a tire according to claim 1, wherein a tap hole and/or a runner which provides the tap hole with molten metal are/is formed inside said chill. 